Collect the sample in a 40-mL glass vial with a screw-cap lined with a TFE-faced septum. Fill the vial to overflowing, ensuring that there are no air bubbles. Add 25 mg of ascorbic acid as a reducing agent to quench the further production of trihalomethanes. Seal the vial and store the sample at 4oC for no longer than 14 days.Prepare a standard stock solution for each trihalomethane by placing 9.8 mL of methanol in a 10-mL volumetric flask. Let the flask stand for 10 min, or until all surfaces wetted with methanol are dry. Weigh the flask to the nearest ±0.1 mg. Using a 100-mL syringe, add 2 or more drops of trihalomethane to the volumetric flask, allowing each drop to fall directly into the methanol. Reweigh the flask before diluting to volume and mixing. Transfer the solution to a 40-mL glass vial with a TFE-lined screwtop and report the concentration in mg/mL. Store the stock solutions at –10 to –20oC and away from the light.